1. Field of the Invention
This invention relates to formulations and processes for swelling, etching and plating for decorative and functional purposes of plastics of functionalized polymers, suitably those having at least one hydrolyzable functionality such as anhydride, carbonate, ester (carboxyl and sulfonic), amide, urethane and imide. The polymer could be a copolymer and/or an alloy of two or more polymers. The invention is of particular value with poly(oxy-carbonyloxy-1,4-phenylene-isopropylidene-1,4-phenylene), referred herein as polycarbonate and/or PC. The etching formulations utilize at least one swelling agent and at least one degradation agent. The etched surface can be plated with electroless nickel or copper with satisfactory adhesion of the plated metals.
2. Brief Description of the Prior Art
General Prior Art
General art of etching and plating of plastics, including formulations and processes is described in a number of books and reviews including: 1. Guidebook Directory, Metals and Plastics Publications, Inc., Hackensack, N.J. 1988; 2. Standards and Guidelines for Electroplated Plastics, American Society of Electroplated Plastics, Englewood Cliffs, N.J. 1984; 3. Metallic Coating of Plastics, W. Goldie, Electrochemical Publication Ltd., Hatch End, England, 1968; and 4. Electroplating of Plastics, I. R. Weiner, Finishing Publications Ltd., Hampton Hill, England, 1977. Plastic parts are subjected to the following major processes to plate them: pre-swelling (if required), etching, neutralizing, catalyzation, acceleration, electroless plating and electroplating. The details of these processes and formulations are given in the above references.
Prior Art for Polycarbonate
Polymeric plastics are plated for decorative and functional reasons such as decoration and electromagnetic interference (EMI) shielding. In order to prevent EMI from escaping the source and protect sensitive electronic equipment from outside EMI sources, electronic equipment which can generate frequency between 10.sup.4 to 10.sup.9 (10,000 to 10 billion) cycles per second should be shielded. The most widely used plastic for housing electronic devices is polycarbonate. Polycarbonate is generally unaffected by aqueous solution of etchants and hence requires pre-swelling. The terms, swelling, pre-swelling, pre-etching, pre-conditioning, and pre-treatment are used interchangeably herein.
Many polymers having hydroxylatable functionalities, such as polycarbonate have tough surfaces. They cannot be sufficiently etched with common aqueous etchants such as aqueous chromic acid or aqueous alkali metal hydroxides. Pre-swelling is required for fast etching. In particular, polycarbonate is not soluble in a number of common organic solvents. The good or "fast" solvents are those in which it is highly soluble at room temperature. The "poor", "slow" or "minor roughening" solvents are those in which it has limited solubility at room temperature. The nonsolvents are those in which it is insoluble.
Good solvents usually cause excessive swelling, hence a surface which is very rough and unsuitable for subsequent plating. If transparent plastic is used, it becomes essentially opaque. With poor solvents which cause a proper degree of swelling, the surface is uniformly micro-roughened and can be plated. If transparent plastic is used, it becomes translucent in a poor solvent. This properly swollen surface is required for proper etching and plating of plastics. Nonsolvents do not swell the plastic and hence there can be no adhesion for plating. Thus, if a transparent sheet is used, it remains transparent.
Preswollen PC is easy to degrade as the swelling provides a higher surface area and porous surface. As a result, the etching/degradation is rapid. The roughened surface also provides a strong mechanical bonding between the plastic and the plated metals to be plated thereon. Degradation of PC provides polar functionalities such as --OH and --COOH on the micro-roughened surface which, in turn, provides a wettable surface. Such a wettable surface is required for electroless plating of metals as all commercially available plating baths are aqueous.
One pre-swelling system available commercially, uses toxic halocarbon solvents for pre-swelling and is frequently followed by chromic acid as an etchant. For example, the pre-swelling system offered by Shipley inc., (according to the Material Safety Data Sheet of Shipley for PM-921 Conditioner) uses dichloropropanol to pre-etch polycarbonate.
There is a need for a nontoxic solvent for pre-etching polycarbonate. There is also a need for a etching system for polycarbonate which is nontoxic and provides excellent adhesion of electroless copper or nickel (at least as good as that obtained with chromic acid). In order to reduce the cost due to drag out and save time, it is desirable to utilize one bath to simultaneously achieve swelling and etching of polycarbonate.
As halocarbons can undergo dehydrohalogenation and oxidation, it is difficult to prepare a stable bath containing a mixture of halocarbon (as pre-etch) with either a strong base etchant or with chromic acid which is a strong oxidizing agent. In addition, chromic acid is toxic, expensive, produces a toxic fumes. Furthermore, recovery of chromium and waste disposal of chromium (sludge) is expensive. While alkali metal hydroxides have been used for etching polycarbonate, they are not very effective in providing a surface with satisfactory properties for electroless plating. The use of mixtures of swelling agents (and/or solubilizing agents) and degradation agents as etching systems for polycarbonate has not been reported.
Prior Patents
A number of patents have been issued on swelling, etching, and plating of plastics. Most of these patents relate to etching of ABS (a copolymer of acrylonitrile, butadiene, and styerene) with chromic acid. There are several patents on pre-swelling and non-chromic acid etching.
U.S. Pat. No. 3,649,391 describes a process for producing a marking receptive (pre-swelling/etching) surface of polystyrene by action of a solution of chloromethane, perchloroethylene, and heptane. The claimed formulations can be used for polystyrene only. U.S. Pat. Nos. 3,660,293, 3,671,289, 3,795,622 and 4,281,034 describe processes of increasing adhesion of electroless metals by pre-swelling ABS with aqueous solutions of hydroxylated and etherified monoacetate, e.g., glycol diacetate and cellusolve acetate; five membered heterocyclic compounds such as propylene carbonate and mixture of halocarbons and etchanol. These patents do not describe the art of pre-swelling polycarbonate or other plastics. A method of increasing adhesion of polyepoxy is presented in U.S. Pat. No. 3,758,332. The surface of polyepoxy is swollen with solvents such as methylethyl ketone, tetrahydrofuran, dioxane and like. Processes of pre-swelling polyamides with aqueous solution of organic acids such as trichloracetic acid and other chlorinated solvents such as dichloropropanol are described in U.S. Pat. Nos. 4,125,649 and 4,335,164. U.S. Pat. No. 4,125,649 also include formulations containing water miscible halocarbons for pre-swelling followed by etching with chromic acid.
A published patent application (PCT Int. Appl. No. 86/US/1199, Jan. 15, 1987) to C. Courduvelis and D. E. Stone, describes formulations and processes of pre-swelling plastics such as polyether polyimide with solvents such as dimethylsulfoxide, propanediol ether, followed by etching with chromic acid.
The prior art describes neither (1) etching of polycarbonate in one step nor (2) etching of polycarbonate in two steps (pre-swelling) followed by degradation wherein polycarbonate is degraded (etched) with a solution containing a solubilizing agent, and/or a wetting agent and a base. Etching pre-swollen polycarbonate with mineral acids other than chromic acid is also not taught.